This invention pertains to a power transmission for vehicles such as, farm and agricultural tractors and having three modes of operation derived from a simple planetary differential wherein low-and high-speed ranges are provided by a variable speed hydraulic transmission input to the differential and an intermediate work speed range by a hydromechanical operation with inputs to the differential from both the hydraulic transmission and a mechanical drive. More specifically, the hydraulic transmission embodies a variable speed hydrostatic transmission.
A two-mode power transmission with a low-speed hydraulic range and a higher speed hydromechanical range is known, as shown in Gunderson U.S. Pat. No. 3,427,899 and Singer U.S. Pat. No. 3,626,787. It is also known to have a hydromechanical low-speed range of operation for a transmission and a higher speed range which is derived from a variable speed hydraulic transmission which drives the differential. These known systems have not disclosed the full utilization of a simple planetary differential unit to provide a low-speed, high torque hydrostatic drive with an intermediate speed range hydromechanical drive for normal work action of the vehicle and a high-speed hydrostatic drive for mobility.